Method of making sound absorbing laminates and laminates having maximized sound absorbing characteristics

ABSTRACT

A sound absorption laminate comprises a porous insulation substrate, such as, a glass, polymeric or natural fiber blanket or a foamed polymeric resin sheet and a facing sheet with a high air flow resistance. The facing sheet is superimposed upon a surface of the porous insulation substrate to augment the acoustical properties of the substrate. With the facing sheet the air flow resistance of the laminate is greater than the air flow resistance of the substrate and the laminate exhibits a higher sound absorption coefficient than the sound absorption coefficient of the substrate. Thus, the laminate exhibits better sound absorption properties than the substrate and is suitable for sound absorption applications for which the substrate alone would not be suitable.

This is a continuation-in-part of application Ser. No. 08/224,600 filedApr. 7, 1994, now U.S. Pat. No. 5,459,291 which is acontinuation-in-part of application Ser. No. 07/953,415 filed Sep. 29,1992, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to the use of facings, having high airflow resistances, to enhance the sound absorption properties of certainporous insulation materials and especially, those insulation materialshaving low air flow resistances. By laminating the high air flowresistance facings to these particular porous insulation materials,these porous insulation materials exhibit sound absorptioncharacteristics normally provided by more costly fibrous insulationmaterials of greater thickness, higher bulk density and/or smalleraverage fiber diameter and foam insulation materials having smallercells and pores.

Porous insulation materials such as thermoplastic glass or polymericfiber blankets and polymeric foams are used in many applications toenhance the sound absorption performance of various products andsystems. Typical applications include: acoustical wall panels, ceilingpanels and office partitions; automotive headliners and hoodliners;liners for heating, ventilating and air conditioning systems; applianceinsulation; and similar applications.

The sound absorption characteristics of these porous insulationmaterials is a function of the acoustic impedance of the material. Theacoustic impedance is a complex quantity consisting of frequencydependent components called, respectively, acoustic resistance andacoustic reactance. The acoustic reactance of these porous insulationmaterials is governed largely by the thickness of the product and, to amuch lesser extent, by the mass per unit area of an air permeable facingor film which may be applied over the surface of the porous insulationmaterial. The acoustic resistance of the porous insulation material isgoverned by the air flow resistance of the porous insulation material.

The ratios of the acoustic reactance and the acoustic resistance to thecharacteristic impedance of air determines the normal incidence soundabsorption coefficient. For a given value of the acoustic reactanceratio, there is an optimum value of the acoustic resistance ratio whichwill provide the maximum sound absorption. Since the reactance ratio ofa porous insulation material is determined largely by the thickness ofthe porous insulation material, the most effective way of controllingthe sound absorption properties of a porous insulation material is byadjusting the acoustic resistance ratio. In the past, the acousticresistance ratio has been adjusted by changing the physical propertiesof the porous insulation materials. In fibrous insulations, such asglass fiber insulations, the average fiber diameter of the insulationhas been decreased, the bulk density of the insulation has beenincreased, and the binder content of the insulation has been increased.In polymeric resin foam insulations, the average pore or cell size ofthe insulation material has been decreased. While these physicalmodifications increase the acoustic resistance ratio of these insulationproduct, the cost of producing these products is also increased.

SUMMARY OF THE INVENTION

The present invention uses a thin, coated or uncoated, semi-porouspaper, fabric or perforated film facing of controlled air flowresistance to increase the air flow resistance of an underlying porousinsulation, such as, a glass or polymeric fiber insulation or apolymeric foam insulation, having an acoustic resistance ratio less thanthe optimum acoustic resistance ratio for optimum sound absorption. Theacoustic reactance ratio of the laminate formed by applying the facingto the porous insulation substrate is not materially different from theacoustic reactance ratio of the porous insulation substrate. However,the increased air flow resistance of the laminate (formed by applyingthe facing to the porous insulation substrate) relative to the air flowresistance of the porous insulation substrate, results in an acousticresistance ratio for the laminate which is greater than the acousticresistance ratio of the porous insulation substrate. Accordingly, thesound absorption properties of the laminate are superior to those of theporous insulation substrate.

The benefits of the present invention are most dramatic when suchfacings are applied to low cost, thin, lightweight fibrous insulationsmade with large diameter fibers and thin, lightweight, polymeric foaminsulations having large cells and pores. The air flow resistanceprovided by such insulations is frequently too low to provide adequatesound absorption for many applications. By increasing the air flowresistance of these low cost porous insulation materials through the useof controlled air flow resistance facings, the sound absorbingproperties of these porous insulation materials are improved so thatthese low cost insulations can be used for more demanding applicationspreviously requiring the use of more expensive insulation materials.

However, the greater the thickness and/or bulk density of a porousinsulation material, the greater the air flow resistance of thematerial. For porous insulation materials of a certain thickness and/ordensity, the air flow resistance of the porous insulation material,alone, provides the insulation with an acoustic resistance ratio at orabove the optimum acoustic resistance ratio for optimum soundabsorption. For these porous insulation materials, increasing the airflow resistance, by applying a facing to the porous insulation materialwill only degrade the sound absorption properties of the insulationmaterial. Thus, the present invention is directed to the use of thin,coated or uncoated, semi-porous, facings, only, on those porousinsulation materials where the air flow resistance of the facing, whencombined with the air flow resistance of the porous insulation material,forms a faced porous insulation laminate with superior sound absorptionproperties to those of the porous insulation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view of a sound absorbing laminate of thepresent invention comprising a thin, semi-porous facing sheet and aporous insulation substrate.

FIGS. 2.1 to 2.6 show the calculated normal incidence sound absorptioncoefficients for nominally one half inch thick glass fiber insulation,at progressively higher bulk densities ranging from 0.5 to 6.0 poundsper cubic foot, with and without an optimum air flow resistance facingapplied to the insulation.

FIGS. 3.1 to 3.6 show the calculated normal incidence sound absorptioncoefficients for nominally one inch thick glass fiber insulation, atprogressively higher bulk densities ranging from 0.5 to 6.0 pounds percubic foot, with and without an optimum air flow resistance facingapplied to the insulation.

FIG. 4 is a cross section of a sound absorbing laminate of the presentinvention comprising a thin, semi-porous facing sheet intermediate twoporous insulation substrates.

FIG. 5 is a cross section of a sound absorbing laminate of the presentinvention wherein the laminate comprises a plurality of porousinsulation substrates and both inner and outer thin, semi-porous facingsheets.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a sound absorption laminate 12 comprising a porousinsulation substrate 14 and a controlled porosity facing 16 superimposedupon one surface of the porous insulation substrate. A preferred meansof maintaining the facing 16 in place on the substrate 14 is to adherethe facing to the major surface of the porous insulation substrate 14,e.g. by adhesive bonding or thermal bonding. However, the facing 16 maybe secured to or maintained in place on the surface of the porousinsulation a substrate by other means, including but not limited to,mechanical fasteners adhering, bonding or otherwise securing the facingto the substrate along the edges or sides of the substrate or byotherwise directly or indirectly securing the facing to the substrate.While the porous insulation substrate 14 can be a relatively highdensity substrate, the porous insulation substrate 14 is typically a lowcost, thin, lightweight, large diameter fiber, fibrous insulationmaterial, such as, a glass fiber, thermoplastic polymeric fiber,thermosetting polymeric fiber, carbonaceous fiber, carbonized fiber,graphitized fiber insulation or a cotton fiber, milkweed fiber or othernatural fiber insulation having a bulk density of less than two poundsper cubic foot, or a low cost, thin, lightweight foam insulationmaterial, such as a polymeric foam, having large cells and pores. Thesesubstrates typically are less than three inches in thickness. Thecontrolled porosity facing 16 is a thin, coated or uncoated, semi-porouspaper, fabric or perforated film facing having a controlled air flowresistance which is selected to add to the air flow resistance of theunderlying substrate 14 whereby the sound absorption properties of thelaminate 12 are improved over the sound absorption properties of thesubstrate 14, alone. The appropriate facing for a particular substrate14 is selected as follows.

The sound absorption performance of the porous insulation substrate 14is a function of the acoustic impedance of the substrate 14. The soundabsorption performance of the sound absorption laminate 12 is a functionof the acoustic impedance of the laminate 12. The acoustic impedance ofthe porous insulation substrate 14 is a function of the acousticresistance and the acoustic reactance of the substrate. The acousticimpedance of the sound absorption laminate 12 is a function of theacoustic resistance and the acoustic reactance of the laminate. Theacoustic reactance of the porous insulation substrate 14 or the soundabsorption laminate 12 is governed largely by the thickness of thesubstrate or the laminate. The acoustic resistance of the porousinsulation substrate 14 or the sound absorption laminate 12 is governedlargely by the air flow resistance of the substrate or the laminate. Theratios of the acoustic reactance and the acoustic resistance to thecharacteristic impedance of air, c, for the porous insulation substrate14 or the laminate 12 determine the sound absorption characteristics ofthe substrate or the laminate. The angle of incidence of sound waveswith respect to the surface of the porous insulation substrate 14 or thesound absorption laminate 12 also affects the degree of sound absorptionachieved by the substrate or the laminate. For the purposes ofillustration, the following discussion deals only with the normal (90°angle of incidence) sound absorption properties of a porous insulationsubstrate or a laminate. However, the concept of enhancing the soundabsorption characteristics of a porous insulation substrate, through theapplication of a high air flow resistance facing to the substrate,applies to products intended to absorb sound in both normal incidenceand/or diffuse (random incidence) sound fields. The followingcalculations of the normal incidence acoustical properties orcharacteristics of porous insulation substrates 14 and laminates 12, aswell as, the procedures for estimating the desired air flow resistancecharacteristics of the high air flow resistance facing to be applied tothe substrate are intended to be used only as a first approximation.

Once the value of the optimum air flow resistance for a facing to beapplied to a given porous insulation substrate has been estimated, theactual value of the optimum air flow resistance for a facing to beapplied to a given porous insulation substrate, to maximize the soundabsorption properties of the laminate formed by applying the facing tothe substrate, is best determined experimentally using a range ofvarious air flow resistance facings over the particular substrate.Optimizing such a sound absorption laminate system for normal incidencesound absorption properties can be accomplished by using either thestanding-wave or the two-microphone impedance tube methods described inASTM methods C-384 and E-1050, respectively. Optimizing the randomincidence sound absorption of a particular sound absorption laminatesystem can be accomplished experimentally by using the reverberationroom test method described in ASTM method C-423.

For normally incident sound, the sound absorption of a substrate orsound absorption laminate may be estimated from the followingrelationship:

    ∝.sub.n =(4r/ρc)/ (r/ρc+1).sup.2 +(x/ρc).sup.2 ! Eq. 1

here:

∝_(n) =normal incidence sound absorption coefficient

r/ρc=acoustic resistance ratio

x/ρc=acoustic reactance ratio

ρc=characteristic impedance of air (approximately 406 mks rayls)

Thus, as shown by Equation 1, for a given value of the acousticreactance ratio (x/ρc) which is governed mainly by the thickness of theporous insulation substrate 14 or sound absorption laminate 12, there isan optimum value of the acoustic resistance ratio (r/ρc) which willprovide the largest normal incidence sound absorption coefficient forthe porous insulation substrate or the sound absorption laminate 12.Where the fibrous insulation substrate 14, due to its thickness, bulkdensity and/or fiber diameters, or the polymeric foam insulationsubstrate 14, due to its thickness and/or pore and cell size, alreadyhas a sufficient air flow resistance to optimize the value of theacoustic resistance ratio, the lamination of a thin, semi-porous facingto the substrate will not enhance the sound absorption characteristicsof the substrate and the facing should not be applied for acousticalpurposes. However, where the porous insulation substrate does not havethe required air flow resistance to optimize the value of the acousticresistance ratio for the particular substrate, a thin, semi-porousfacing laminated to the substrate can increase the air flow resistanceof the substrate to optimize the value of the acoustic resistance ratioand thereby optimize the sound absorption coefficient.

To determine whether or not a facing can be laminated to a porousinsulation substrate 14 to improve the acoustical performance of thesubstrate, the acoustic resistance ratio of the unfaced substrate, theacoustic reactance ratio of the unfaced substrate and the optimumacoustic resistance ratio for the substrate should be calculated. If thevalue of the acoustic resistance ratio of the unfaced substrate isalready sufficient to maximize the sound absorption properties of thesubstrate, there is no need to apply a facing to the substrate foracoustical purposes. If the value of the acoustic resistance ratio ofthe porous insulation substrate is insufficient to maximize the soundabsorption properties of the substrate, the additional air flowresistance to be provided by a facing to achieve the optimum acousticresistance ratio is determined and the appropriate facing is selected.

Equations 2, 3, and 4, below, can be used to determine the acousticresistance ratios and the acoustic reactance ratios for the porousinsulation substrate 14 and the sound absorption laminate 12. Theseequations enable one to determine whether or not a facing can enhancethe acoustical performance of a porous insulation substrate and, if theacoustical performance can be enhanced, the additional air flowresistance required from a facing to optimize the acoustic resistanceratio.

For plain glass fiber insulations without a facing, the acousticresistance of the substrate (r_(s)) may be either measured directly byASTM method C-522 or estimated on the basis of the following empiricalrelationship:

    r.sub.s =3180(ρ.sub.s.sup.1.43 /d.sup.2)t              Eq. 2

where:

r_(s) =the air flow resistance of the substrate in mks rayls

ρ_(s) =the glass fiber insulation bulk density in kg/m³

d=the glass fiber mean diameter in microns

t=the thickness of the glass fiber insulation in meters

The acoustic resistance ratio of a laminate of the glass fiberinsulation, with a facing applied, is calculated as follows. Theacoustic resistance (r_(s)) of the glass fiber insulation substrate iseither measured or computed as in Equation 2, above. The additionalacoustic resistance provided by the facing (r_(f)) is added to theacoustic resistance of the glass fiber insulation substrate to obtainthe acoustic resistance of the sound absorption laminate. This sum isthen divided by the characteristic impedance of air (ρc) to obtain theacoustic resistance ratio (r_(L) /ρc) of the sound absorption laminateas follows:

    r.sub.L /ρc=(r.sub.s +r.sub.f)/ρc                  Eq. 3

where

r_(L) /ρc=the acoustic resistance ratio of the glass fiber insulationsubstrate plus the facing, dimensionless

r_(f) =the acoustic resistance of the facing in mks rayls

r_(s) =the acoustic resistance of the glass fiber insulation substratein mks rayls

The acoustic reactance ratio (x/ρc) used in the calculation of thenormal incidence sound absorption coefficient can be approximated by thefollowing expression:

    x/c=-1  1/tan (2πfL/c)!                                 Eq. 4

where

x/ρc=the acoustic reactance ratio, dimensionless

π=3.14159

f=frequency, Hz.

L=the distance from the surface of the outer face of the glass fiberinsulation substrate to a reflective backing behind the substrate, suchas a rigid wall, in meters

c=the speed of sound in meters per second

At a given frequency, the optimum acoustic resistance ratio for a soundabsorption laminate, r_(L) /ρc, will be approximately equal to thefollowing expression:

    r.sub.L /ρc= (1+(x/ρc).sup.2 !.sup.0.5             Eq. 5

In order to determine whether or not the addition of a high flowresistance facing will improve the normal incidence sound absorptionprovided by a particular porous insulation substrate at a givenfrequency, the acoustic resistance ratio (r_(s) /ρc) of the substrate ismeasured (ASTM C-522) or computed (Eq. 2) and the acoustic reactanceratio (x/ρc) is computed (Eq. 4). If the magnitude of the acousticresistance ratio (r_(s) /ρc) of the porous insulation substrate isnumerically less than the optimum value computed from Equation 5, theapplication of a high flow resistance facing to the porous insulationsubstrate will likely improve the normal incidence sound absorption ofthe substrate. If the magnitude of the acoustic resistance ratio of theporous insulation substrate is numerically equal to or larger than theoptimum value computed from Equation 5, the application of a high flowresistance facing to the substrate will likely reduce the normalincidence sound absorption provided by the substrate alone.

This procedure can be repeated over a range of frequencies in order todetermine whether or not a high air flow resistance facing will bebeneficial and, if so, approximately what value of flow resistance isrequired for the facing.

The desired value of acoustic resistance ratio for the facing materialto be applied over a particular substrate will then be the differencebetween the acoustic resistance ratio of the porous insulation substrateand the optimized value of the acoustic resistance ratio for thesubstrate. Thus, the acoustical properties of thin, low density, porousinsulation materials can be upgraded through the use of the appropriatethin, coated or uncoated, semi-porous paper, fabric or perforated filmfacing.

FIGS. 2.1 to 2.6 and 3.1 to 3.6 illustrate how the use of a thin,semi-porous facing can enhance the sound absorption characteristics ofthin, low density, porous glass fiber insulation materials. FIGS. 2.1 to2.6 show the calculated normal incidence sound absorption coefficientsfor frequencies from 100 to 5,000 Hz. for one-half inch nominalthickness faced and unfaced glass fiber insulation, comprising fibershaving a mean fiber diameter of 4.7 microns, at bulk densities of 0.5,1.0, 1.5, 2.0, 4.0 and 6.0 pounds per cubic foot. FIGS. 3.1 to 3.6 showthe calculated normal incidence sound absorption coefficients forfrequencies from 100 to 5,000 Hz. for one inch nominal thickness facedand unfaced glass fiber insulation, comprising fibers having a meanfiber diameter of 4.7 microns, at bulk densities of 0.5, 1.0, 1.5, 2.0,4.0 and 6.0 pounds per cubic foot. As illustrated in FIGS. 2.1 to 2.6, afacing can greatly enhance the sound absorption characteristics forone-half inch thick glass fiber insulation for densities up toapproximately 4.0 pounds per cubic foot. Above a density of 4.0 poundsper cubic foot, the flow resistance of the porous insulation has alreadyreached the optimum value and a facing does not augment the acousticalproperties of the glass fiber insulation. As shown in FIGS. 3.1 to 3.6,a facing can greatly enhance the sound absorption characteristics forone inch thick glass fiber insulation for densities up to approximately1.5 pounds per cubic foot. Above a density of 1.5 pounds per cubic foot,the flow resistance of the porous insulation has already reached theoptimum value and a facing does not augment the acoustical properties ofthe glass fiber insulation. Thus, FIGS. 2.1 to 2.6 and 3.1 to 3.6illustrate that the present invention, through the use of relativelyinexpensive, thin, semi-porous facings can upgrade the performance ofparticular inexpensive, porous insulations whereby such inexpensiveinsulations can be used for more demanding applications previouslyrequiring the use of more expensive insulation materials.

In FIGS. 2.1 to 2.6 and 3.1 to 3.6, the optimum acoustic resistancevalue for a particular insulation of a given thickness and bulk densitywas calculated by iteratively computing the normal incidence soundabsorption coefficient for that thickness and density as a function ofr_(f). The value of r_(f) which provided the highest average normalincidence sound absorption coefficient for the frequencies of 250, 500,1,000 and 2,000 Hz. was taken as the optimum air flow resistance valuefor the facing. The frequencies of 250, 500, 1,000 and 2,000 Hz wereselected for determining the optimum air flow resistance for the facingbecause sound absorptive materials are normally specified on the basisof the single number Noise Reduction Coefficient (NRC). The NRC iscomputed on the basis of the average random incidence sound absorptionat those four frequencies. It is assumed that the random incidence soundabsorption coefficients will rank in order with the normal soundincidence coefficients as computed for FIGS. 2.1 to 2.6 and 3.1 to 3.6.

This assumption was verified using Manville one-half inch nominalthickness, 1.5 pound per cubic foot, EXACT-O-COTE glass fiber insulationwith and without a high air flow resistance SNOWWEB fibrous fabricfacing imbedded in an acrylic coating applied to one side of the glassfiber insulation. With the use of the SNOWWEB facing, the air flowresistance of the sound absorption laminate was approximately 740 mksrayls while the air flow resistance of the glass fiber insulation,alone, was only approximately 360 mks rayls. The average normalincidence sound absorption coefficient for the frequencies of 250, 500,1,000 and 2,000 Hz. was increased from 0.29 to 0.37 and the randomincidence noise reduction coefficient (NRC) was increased from 0.55 to0.60 even though the flow resistance for the laminate was below thetheoretical optimum value of approximately 1,250 mks rayls. Toapplicants' knowledge, a 0.60 NRC value was not previously possible toattain in an one-half inch thick glass fiber insulation product at abulk density of approximately 1.5 pounds per cubic foot.

FIG. 4 shows a sound absorption laminate 112 of the present inventioncomprising porous insulation substrates 114 and 116 and a controlledporosity facing 118 intermediate major surfaces of the porous insulationsubstrates 114 and 116. Like the porous insulation substrate 14 of FIG.1, the porous insulation substrates 114 and 116 are typically a lowcost, thin, lightweight, large diameter fiber, fibrous insulationmaterial, such as, a glass fiber, thermoplastic polymeric fiber,thermosetting polymeric fiber, carbonaceous fiber, carbonized fiber,graphitized fiber insulation or a cotton fiber, milkweed fiber or othernatural fiber insulation having a bulk density of less than two poundsper cubic foot, or a low cost, thin, lightweight foam insulationmaterial having large cells or pores, such as a polymeric foaminsulation. Like the controlled porosity facing 16, the controlledporosity facing 118 is a thin, coated or uncoated, semi-porous paper,fabric, or perforated film facing having a controlled air flowresistance which is selected to add to the air flow resistance of one orboth of the substrates 114 and 116 whereby the sound absorptionproperties of the laminate 112 is improved over the sound absorptionproperties of the substrates 114 and 116 alone. The appropriate facing118 for either substrate 114 or 116 or both substrates 114 and 116,considered together, is selected the same way the facing 16 is selectedfor the substrate 14 to form the laminate 12 of FIG. 1.

A preferred means of maintaining the facing 118 in place, as an innerfacing or septum, between the substrates of the laminate 112 is toadhere the facing 118 to the major surfaces of the porous insulationsubstrates 114 and 116, e.g. by adhesive or thermal bonding. However,the facing 118 may be secured to or maintained in place between themajor surfaces of the porous insulation substrates 114 and 116 by othermeans, including but not limited to, mechanical fasteners, adhering orotherwise securing the facing 118 to the substrates along the edges orsides of the substrates or by otherwise directly or indirectly securingthe facing to or maintaining the facing in place between the substrates.

FIG. 5 shows a sound absorption laminate 212 of the present inventioncomprising multiple layers of substrates 214, 216 and 218 and controlledporosity facings 220, 222 and 224. Facing 220 is superimposed uponsubstrate 214 and facings 216 and 218 are intermediate substrates220-222 and 222-224, respectively.

Like the porous insulation substrate 14 of FIG. 1, the porous insulationsubstrates 214, 216 and 218 are typically a low cost, thin, lightweight,large diameter fiber, fibrous insulation material, such as, a glassfiber, thermoplastic polymeric fiber, thermosetting polymeric fiber,carbonaceous fiber, carbonized fiber, graphitized fiber insulation or acotton fiber, milkweed fiber or other natural fiber insulation having abulk density of less than two pounds per cubic foot, or a low cost,thin, lightweight foam insulation material having large cells or pores,such as, a polymeric foam insulation. Like the controlled porosityfacing 16 of FIG. 1, the controlled porosity facings 220, 222 and 224are thin, coated or uncoated, semi-porous paper, fabric or perforatedfilm facings having controlled air flow resistances which are selectedto add to the air flow resistance of an associated substrate orsubstrates whereby the sound absorption properties of the laminate 212(including one or more layered portions, e.g. facing 220 and substrate214; facing 222 and substrate 216; or facing 224 and substrate 218 ofthe laminate) are improved over the sound absorption properties of thesubstrates 214, 216 and 218 alone. Thus, in a preferred embodiment ofthe laminate of FIG. 5, the air flow resistances of the substrates andthe facings can be selected so that the laminate 212, as a whole, has anacoustic resistance ratio that is maximized around the optimizedacoustic resistance ratio of the substrates or the facings can togethercan have an acoustic resistance ratio that is about equal to thedifference between the acoustic resistance ratio of the substrates andthe optimized value of the acoustic resistance ratio for the substrates.The appropriate facings for the substrates of the laminate 212 areselected the same way the facing 16 is selected for the substrate 14 toform the laminate 12 of FIG. 1.

A preferred means of maintaining the facing 220 in place upon substrate214 and facings 222 and 224 in place as inner facings or septums,between substrates 214-216 and 216-218, respectively, is to adhere thefacings to the major surfaces of the porous insulation substrates e.g.by adhesive or thermal bonding.

However, the facings 220, 222 and 224 may be secured to or maintained inplace upon and between the major surfaces of the substrates 214, 216 and218 by other means, including but not limited to, mechanical fasteners,adhering or otherwise securing the facing to the substrates along theedges or sides of the substrates or otherwise directly or indirectlysecuring the facing to or maintaining the facings in place upon orbetween the substrates.

In describing the invention certain embodiments have been used toillustrate the invention and the practice thereof. However, theinvention is not limited to these specific embodiments as otherembodiments and modifications within the spirit of the invention willreadily occur to those skilled in the art on reading this specification.Thus, the invention is not intended to be limited to the specificembodiments disclosed, but is to be limited only by the claims appendedhereto.

What is claimed is:
 1. A method of making a sound absorbing laminatecomprising:a) selecting a porous insulation substrate having low airflow resistance; b) determining the optimized value of the acousticresistance ratio for said substrate; c) selecting a thin, semi-porousfacing having an acoustic resistance ratio such that when superimposedupon a face of said substrate, a laminate is formed having an acousticresistance ratio greater than the acoustic resistance ratio of saidsubstrate and no greater than the optimized value of the acousticresistance ratio for said substrate; and d) superimposing said facing onsaid substrate to form a laminate having good sound absorbingcharacteristics.
 2. The method of claim 1, including: adhering saidfacing to said face of said substrate.
 3. The method of claim 1,wherein: said substrate is comprised of a material selected from thegroup of glass fibers, thermoplastic polymeric fibers, thermosettingpolymeric fibers, cotton fibers, milkweed fibers, carbonaceous fibers,carbonized fibers, graphitized fibers and a polymeric foam.
 4. Themethod of claim 1, wherein: said facing selected has an acousticresistance ratio that when superimposed upon said face of saidsubstrate, a laminate is formed having an acoustic resistance ratio thatis approximates or equals the optimized value of the acoustic resistanceratio for said substrate.
 5. The method of claim 4, wherein: saidsubstrate is comprised of a material selected from the group of glassfibers, thermoplastic polymeric fibers, thermosetting polymeric fibers,cotton fibers, milkweed fibers, carbonaceous fibers, carbonized fibers,graphitized fibers, and a polymeric foam.
 6. The method of claim 1,wherein: said facing selected has an acoustic resistance ratio thatapproximates or equals the difference between the acoustic resistanceratio of said substrate and the optimized value of the acousticresistance ratio for said substrate.
 7. The method of claim 6, wherein:said substrate is comprised of a material selected from the group ofglass fibers, thermoplastic polymeric fibers, thermosetting polymericfibers, cotton fibers, milkweed fibers, carbonaceous fibers, carbonizedfibers, graphitized fibers, and a polymeric foam.
 8. A laminate havingmaximized sound absorbing characteristics, comprising:a) a porousinsulation substrate having an air flow resistance less than thatrequired to produce an optimum sound absorbing characteristic for saidsubstrate; and b) a thin, semi-porous facing superimposed upon a face ofsaid substrate, said facing having an acoustic resistance ratio thatwhen superimposed upon said face of said substrate, a laminate is formedhaving an acoustic resistance ratio that approximates or equals theoptimized value of the acoustic resistance ratio for said substrate. 9.The laminate of claim 8, wherein: said facing is secured to saidsubstrate.
 10. The laminate of claim 8, wherein: said facing is adheredto said face of said substrate.
 11. The laminate of claim 8, wherein:said substrate is comprised of a material selected from the group ofglass fibers, thermoplastic polymeric fibers, thermosetting polymericfibers, cotton fibers, milkweed fibers, carbonaceous fibers, carbonizedfibers, graphitized fibers and a polymeric foam.
 12. A laminate havingoptimized sound absorbing characteristics comprising:a) a porousinsulation substrate having an air flow resistance less than thatrequired to produce an optimum sound absorbing characteristic for saidsubstrate; and b) a thin, semi-porous facing superimposed upon a face ofsaid substrate, said facing having an acoustic resistance ratio thatapproximates or equals to the difference between the acoustic resistanceratio of said substrate and the optimized value of the acousticresistance ratio for said substrate.
 13. The laminate of claim 12,wherein: said facing is secured to said substrate.
 14. The laminate ofclaim 12, wherein: said facing is adhered to said face of saidsubstrate.
 15. The laminate of claim 12, wherein: said substrate iscomprised of a material selected from the group of glass fibers,thermoplastic polymeric fibers, thermosetting polymeric fibers, cottonfibers, milkweed fibers, carbonaceous fibers, carbonized fibers,graphitized fibers and a polymeric foam.